PKS B1718-649: An H I and H2 perspective on the birth of a compact radio source.

We present neutral hydrogen (H I) and warm molecular hydrogen (H2) observations of the young (102 yr) radio galaxy PKS B1718-649. We study the morphology and the kinematics of both gas components, focusing, in particular, on their properties in relation to the triggering of the radio activity. The regular kinematics of the large scale H i disk, seen in emission, suggests that an interaction event occurred too long ago to be responsible for the recent triggering of the radio activity. In absorption, we detect two absorption lines along the narrow line of sight of the compact ( r < 2 pc) radio source. The lines trace two clouds with opposite radial motions. These may represent a population of clouds in the very inner regions of the galaxy, which may be involved in triggering the radio activity. The warm molecular hydrogen (H2 1-0 S(1) ro-vibrational line) in the innermost kilo-parsec of the galaxy appears to be distributed in a circum-nuclear disk following the regular kinematics of the H I and of the stellar component. An exception to this behaviour arises only in the very centre, where a highly dispersed component is detected. These particular H I and H2 features suggest that a strong interplay between the radio source and the surrounding ISM is ongoing. The physical properties of the cold gas in the proximity of the radio source may regulate the accretion recently triggered in this AGN. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]